Adélie penguins and environmental change.

IN THEIR REVIEW “ENVIRONMENTAL CHANGE and Antarctic seabird populations,” (Special Issue on Polar Science, 30 Aug., p. 1510), J. P. Croxall et al. describe paradoxes, many of which exist only because of a limited review of available information. Stating that the paleoecological record shows close correspondence between ice cover and the presence of Adélie penguins is correct (1), but claiming this species’ absence from Antarctica until after the Last Glacial Maximum (LGM) is not supported, particularly by their cited references [(9, 10) in the Review]. These references indicate only that the species spread north to the tip of the Antarctic Peninsula during the Little Ice Age (responding to more persistent sea-ice cover) and has since been retreating, especially during this era of rapid warming (2). Adélie penguin genetics indicate strains that likely resulted from two refugial populations surviving the LGM (3). Croxall et al. mislead by stating that, as a general rule, reduction in sea-ice extent would cause population decline in three pagophilic bird species. This is true, but in the case of Adélie penguins, the best studied of the three, only along the northwestern Antarctic Peninsula (~5% of the Antarctic coast) has the population noticeably declined in the past 50 years (2). Elsewhere during this era of alleged widespread decline in sea-ice extent, this species has been increasing (4). Moreover, once ice persistence decreases, extensive coastlines are available to be colonized, even recolonized– about half the Antarctic circumference (5). Given the recent rapid disappearance of ice shelves in the Antarctic Peninsula region (6), exposing low-lying coastal terrain, new colonies should be forming. Croxall et al. repeatedly claim a widespread decline in the modern presence of sea ice. There are no supporting data for this claim, except for the northern Bellingshausen and Amundsen seas (7–9). A claim exists that the positions of whaling vessels, as affected by sea-ice extent, indicate a large (2.8° latitude), widespread retreat of sea-ice extent just before the satellite, remote-sensing era began (10). Unfortunately, those data are muddled by the economic realities of whaling; i.e., depletion of open-water species required a shift to the pagophilic minke whale (Balaenoptera bonarensis) during the key time period (11), thus bringing ships closer to the pack ice. Comparison of ship positions to satellite-sensed ice edges during summer (the whaling season) shows consistently further north positions of the ice edge (12). Applying this correction to ice edges derived from presatellite era data explains the discrepancy inferred from the ship positions (13). In addition, the current winter sea-ice boundary is within the spatial variation evident during the Holocene, as judged by prevalence of packice–related diatoms in deep-sea cores (14). Finally, Croxall et al. claim repeatedly that the Adélie penguin depends on Antarctic krill (Euphausia superba). If true, arguments later in their paper would apply because E. superba prevalence has been affected inversely by winter sea-ice cover, although only in the northern Bellingshausen Sea (15) where, as noted above, ice persistence has become sporadic. However, such dependence occurs only in that region during summer. Elsewhere, this penguin’s summer diet combines E. superba, E. crystallorophias, and, especially, the fish Pleuragramma antarcticum; in 11 studies quantifying summer diet, the contribution of E. superba averages 40% (range 0 to 99%) (1). In the only study of winter diet [(8) in the Review], which they cite in a misleading way, E. superba contributed just 28% (the remainder: fish and squid). Therefore, the species adjusts its diet depending on prey availability, and diet should not be part of the discussion. Croxall et al. end by asking for a fuller understanding of the climatic and environmental processes affecting the Antarctic fauna. We fully agree, but this requires full appreciation of the available information. D. G. AINLEY,1 G. BALLARD,2 S. D. EMSLIE,3 W. R. FRASER,4 P. R. WILSON,5 E. J. WOEHLER6 1H. T. Harvey & Associates, 3150 Almaden Expressway, Suite 145, San Jose, CA 95118, USA. 2Point Reyes Bird Observatory, Stinson Beach, CA 94970, USA. 3Department of Biological Sciences, University of North Carolina, Wilmington, NC 28403, USA. 4Polar Ocean Research Group, Sheridan, MT 59749, USA. 5LandCare Research NZ, Nelson, Private Bag 6, New Zealand. 6Australian Antarctic Division, Channel Highway, Hobart 7050, Australia.